23 research outputs found
The PL calibration for Milky Way Cepheids and its implications for the distance scale
The rationale behind recent calibrations of the Cepheid PL relation using the
Wesenheit formulation is reviewed and reanalyzed, and it is shown that recent
conclusions regarding a possible change in slope of the PL relation for
short-period and long-period Cepheids are tied to a pathological distribution
of HST calibrators within the instability strip. A recalibration of the
period-luminosity relation is obtained using Galactic Cepheids in open clusters
and groups, the resulting relationship, described by log L/L_sun =
2.415(+-0.035) + 1.148(+-0.044)log P, exhibiting only the moderate scatter
expected from color spread within the instability strip. The relationship is
confirmed by Cepheids with HST parallaxes, although without the need for
Lutz-Kelker corrections, and in general by Cepheids with revised Hipparcos
parallaxes, albeit with concerns about the cited precisions of the latter. A
Wesenheit formulation of Wv = -2.259(+-0.083) - 4.185(+-0.103)log P for
Galactic Cepheids is tested successfully using Cepheids in the inner regions of
the galaxy NGC 4258, confirming the independent geometrical distance
established for the galaxy from OH masers. Differences between the extinction
properties of interstellar and extragalactic dust may yet play an important
role in the further calibration of the Cepheid PL relation and its application
to the extragalactic distance scale.Comment: Accepted for Publication (Astrophysics & Space Science
A Homogeneous Sample of Sub-DLAs I: Construction of the Sample and Chemical Abundance Measurements
In this first paper of a series, we report on the use of quasar spectra
obtained with the UVES high resolution spectrograph and available through the
ESO VLT archive to build the first sample of sub-DLA systems, absorbers with HI
column densities > 10^{19} cm^{-2} but lower than the classical definition of
damped Ly-alpha systems (DLAs) 2x10^{20} cm^{-2}. A systematic investigation of
the properties of these systems and a comparison with those of the DLAs (Paper
II of this series; Peroux et al. 2003b) is expected to provide new clues on the
association of high column density absorbers with galaxies and on the overall
evolution of the neutral hydrogen gas mass and metal content in the Universe.
In the spectra of 22 quasars which were found suitable for a sub-DLA search, we
identified 12 sub-DLAs and 1 borderline case between the DLA and sub-DLA
systems in the redshift interval z = 1.8-4.3. We measured the column densities
of HI and up to 16 ions of low-, intermediate- and high-ionization. We further
investigated the significance of the ionization corrections in the
determination of the chemical abundances from the low-ionization ions in the
sub-DLA HI column density range. Using the predictions of different ion ratios
as a function of the ionization parameter computed with the CLOUDY software
package, we have estimated that with the exception of one case, the ionization
corrections to the abundances of 9 systems for which we were able to constrain
the ionization parameter, are lower than 0.2 dex for all of the elements except
AlII and ZnII down to HI column densities of log N(HI) = 19.3 cm^{-2}. We
finally present the first sub-DLA chemical abundance database which contains
the abundance measurements of 11 different elements (O, C, Si, N, S, Mg, Al,
Fe, Ni, Zn, and Cr).Comment: 35 pages, 43 figures, Accepted for publication in MNRAS
(high-resolution figures available on request from the authors or in the
journal
Studies of Neutron Stars at Optical/IR Wavelengths
In the last years, optical studies of Isolated Neutron Stars (INSs) have expanded from the more classical rotation-powered ones to other categories, like the Anomalous X-ray Pulsars (AXPs) and the Soft Gamma-ray Repeaters (SGRs), which make up the class of the magnetars, the radio-quiet INSs with X-ray thermal emission and, more recently, the enigmatic Compact Central Objects (CCOs) in supernova remnants. Apart from 10 rotation-powered pulsars, so far optical/IR counterparts have been found for 5 magnetars and for 4 INSs. In this work we present some of the latest observational results obtained from optical/IR observations of different types of INSs
Detection of Membrane Protein Two-Dimensional Crystals in Living Cells
It is notoriously difficult to grow membrane protein crystals and solve membrane protein structures. Improved detection and screening of membrane protein crystals are needed. We have shown here that second-order nonlinear optical imaging of chiral crystals based on second harmonic generation can provide sensitive and selective detection of two-dimensional protein crystalline arrays with sufficiently low background to enable crystal detection within the membranes of live cells. The method was validated using bacteriorhodopsin crystals generated in live Halobacterium halobium bacteria and confirmed by electron microscopy from the isolated crystals. Additional studies of alphavirus glycoproteins indicated the presence of localized crystalline domains associated with virus budding from mammalian cells. These results suggest that in vivo crystallization may provide a means for expediting membrane protein structure determination for proteins exhibiting propensities for two-dimensional crystal formation